The use of phase change materials or PCM's for thermal storage has long been known. In many materials much more heat can be stored as latent heat of phase change than as sensible heat. One of the earliest known applications of this principle is the use of ice as a thermal storage medium for perishable foods. Some of the phase transitions that involve significant amounts of latent heat are solid-to-liquid or liquid-to-gas phase changes or, in certain materials, solid-to-solid phase changes. Another subgroup of PCM's uses reversible exothermic reactions, such as hydration-dehydration or solution-precipitation reactions, for heat storage. The latent heat of phase change can be used for heating or cooling depending on whether the phase change is exothermic or endothermic. In most materials, the phase changes are reversible so that the latent heat storage can be used for either heating or cooling depending on the temperature conditions.
PCM's have recently been applied to personal heating and cooling devices. U.S. Pat. No. 4,856,294 to Scaringe et al. describes a Micro-Climate Control Vest which contains a PCM with a solid-to-liquid phase change between 60 and 90° F. as a cooling medium. The vest may also have an optional second PCM layer of ice and an optional outer insulation layer. Because of the inherent rigidity of the PCM in its solid state, the inner liner which contains the PCM is divided into individual compartments separated by vertical seams that provide folding points in the vest. This provides very limited flexibility in the vest, making it impractical for use during intense physical activity where the rigidity would be a hindrance.
U.S. Pat. No. 4,894,931 to Senee et al. describes a Heating Device for warming body parts, such as the soles of the feet. The device uses a battery powered electric heater and a PCM salt. The PCM salt serves as a heat storage medium and as a temperature regulator for the resistance heater since it can absorb a lot of heat without rising above its melt temperature. This device is described as having a rigid base, which along with the inherent rigidity of the solid PCM salt, would make it unusable for applications where flexibility is important.
U.S. Pat. Nos. 4,572,158 to Fiedler and U.S. Pat. No. 4,077,390 to Stanley et al. both describe heating pads for warming body parts that use a supercooled PCM salt solution for heat storage. The PCM can be dissolved by placing the pad in boiling water. Once the PCM is liquefied, it can be cooled to room temperature without solidifying. A trigger is used to activate the PCM, causing an exothermic crystallization which releases heat at about 135° F. The device is sold with a cloth or neoprene cover to prevent burns when it is placed against the skin. These heating pads are made in small sizes for spot heating parts of the body. Because the solid PCM salt is very rigid, the pad cannot be used for heating large areas of the body where the rigidity would be a hindrance to movement.
U.S. Pat. No. 4,756,958 to Bryant et al. describes a fiber with enhanced thermal storage properties and fabrics made therefrom. The fiber described contains microcapsules of PCM ranging from 1-10 microns in size. While these fibers provide the sought after flexibility, they do not contain enough PCM to have sufficient thermal mass to be of practical use in heating or cooling.
U.S. Pat. No. 4,851,291 to Vigo et al. describes another method of making fibers with thermal storage properties by filling the core of a hollow fiber with a PCM or absorbing a PCM onto the surface of a non-hollow fiber. The PCM's described include cross-linked polyethylene glycol and plastic crystals that have a solid-to-solid crystalline phase change. These fibers can also can be made very flexible, but their geometry does not allow absorption of enough PCM into the containment material to be of practical use in heating or cooling.